The storage of granular materials in silos and their discharge through small orifices in the container bottom has been studied widely since more than one century. The discharge dynamics of hard
particles is practically independent of the pressure in the granular bed [1]}, [2]}, [3]}.
The outflow rate \(Q\)  of spheres is well described by Beverloo's
equation [4]}, [5]}. In two dimensions, it reads
\(Q = C \mu \sqrt{g}(W-kD)^{1.5},\) 
